1. Field of the Invention
The present invention relates to a reflector having good reflecting characteristics and a liquid crystal display, which is equipped with the reflector and has good display characteristics, and a method of manufacturing them.
2. Description of the Related Art
There is known a reflection type liquid crystal display having a reflector provided in its inside which reflects incidental light to provide a display light. The reflection type liquid crystal display does not need a backlight as a light source. Therefore, the reflection type liquid crystal display has advantages, such as achieving lower power consumption and a thinner size, over a transmission type liquid crystal display. With those features, the reflection type liquid crystal display is used in a portable terminal or the like. A so-called transflective type liquid crystal display which has the capabilities of both the reflection type and the transmission type is also used in a portable phone or the like. Although the following discussion will describe the problems of the reflection type liquid crystal display, the transflective type liquid crystal display has similar problems.
The reflection type liquid crystal display has a liquid crystal filled in a liquid crystal cell, a switching element for driving the liquid crystal and a reflector provided inside or outside the liquid crystal cell. The reflection type liquid crystal display is, for example, an active matrix type liquid crystal display that uses switching elements, such as thin film transistors.
As a reflection type liquid crystal display, a liquid crystal display which has an undulation shape formed on the surface of a reflection electrode to improve the visibility has been developed. The reflection electrode, when having an undulated surface rather than a flat one, reflects incidental light in multiple directions. That is, forming an undulation shape on the surface of the reflection electrode may improve the display characteristics, such as a wider view angle.
While the undulation shape of the reflection electrode may increase the scattering characteristics of reflected light, there is a case where the interference of the reflected light causes darkening of the screen when the undulation shape has high regularity. To suppress the interference of light, therefore, it is desirable to form the undulation shape having as low regularity as possible.
As one method for providing an undulation shape on the surface of the reflection electrode, a method of forming an undulation shape on the surface of an insulating film has been exploited. In this method, a photosensitive resin film is formed first, which is exposed using an exposure mask and then developed, to form discontinued protruding patterns. Thereafter, the surface of the film is melted by heat treatment, thereby being formed a gentler shape. Then, an organic insulating film is formed on the resin film, being etched for a contact hole thereafter. Finally, a reflection electrode is formed on the insulating film. The undulation shape that is originated from the resin film and the insulating film is formed on the surface of the obtained reflection electrode.
According to the above-described method of forming the undulation shape, the undulation of the insulating film is formed with approximately a constant height, that is, the height of the undulation has substantially two values, because all of the protrusion of the resin film have substantially same height (thickness) value. Here, the height of the undulation means the difference between the height levels (depths) of the top portion and the bottom portion of the undulation in the normal direction of the reflector.
There is developed another method using so-called halftone mask, which method is described in Unexamined Japanese Patent Application KOKAI Publication No. 2000-250025. According to the method, the resin film is patterned using the halftone mask, which has different transmittance in its masking area, so that the protrusions are formed with different height values. However, the number of the height values are substantially two, therefore, the height of the undulation formed on the insulation film has three values.
As explained above, conventionally, the height of the undulation was so set as to have three values at most. Therefore, the undulation shape of the conventional reflector had relatively high regularity and was rather monotonous.
The high regularity of the undulation shape does not provide a good reflecting characteristics and display characteristics. Therefore, the conventional reflector, which was restricted in the number of available values of the height of the undulation of the insulating film, did not have sufficiently improved display characteristics.
Moreover, the above method of forming an undulation on an insulating film requires relatively many steps, that is, formation of two organic films (the resin film and the insulation film), and exposure and development. Further, the undulation formed by using the photolithography technique has a sharp shape, thus requiring a following heat treatment step to make the surface shape gentler. Therefore, the conventional method that uses the photolithography technique has a shortcoming of involving a relatively large number of steps.
As explained above, the conventional reflector had problems such that the undulation shape of the reflection electrode, particularly, the height, had a relatively high regularity so that a sufficiently high reflecting characteristics may not be achieved. Further, the manufacturing method for this reflector had such a problem as to require a relatively large number of steps.